This invention relates in general to a piston for a pressurized container and more particularly to one that is adapted to be employed with a product, such as ice cream, whose flowability varies from a highly flowable state when being loaded into the can to a relatively rigid state when frozen and a somewhat intermediate flowable state when being dispensed.
Pressure operated dispensing containers which employ a piston that is longitudinally slidable within the container are known in the art. These pressurized containers are used to dispense a variety of different materials having different flowability characteristics and varying viscosities. The containers generally are a cylindrical can closed at the bottom end and having a dispensing nozzle and discharge valve at the upper end.
The piston within the container separates the interior of the container into two chambers. The product to be dispensed occupies the upper chamber and pressurized fluid, which acts as a propellant, occupies the lower chamber on the underside of the piston. The piston is generally in the form of an inverted cup and has an upper surface and an annular skirt or sidewall which extends down from the upper surface. The piston, and in particular its upper surface, acts as a barrier wall to separate the product from the propellant. The annular sidewall of the piston stabilizes and positions the piston in the container and provides the surface which rides on the inner wall of the container.
The product to be dispensed is loaded into the upper chamber of the container. After loading the product, an outlet valve is closed. Then propellant is charged into the lower chamber to create a pressure forcing the piston up against the product. When the valve at the top of the container is opened, the propellant pushes the piston towards the top of the container forcing the product to exit the container through the valve and nozzle.
After the container is loaded with product and the piston is pressurized, the piston sidewall and the inner surface of the can wall must maintain a relationship that serves the triple purpose of (a) permitting the piston to ride up as product is dispensed, (b) minimizing the amount of product that seeps down past the clearance between piston sidewall and can sidewall, and (c) minimizing the diffusion of propellant from below the piston around the sidewall into the product. Further, during the dispensing of product, it is important that the piston move smoothly to prevent blow-by of propellant that might occur if the piston sticks in place.
Pistons that Applicant has designed are disclosed in U.S. Pat. No. 4,913,323 issued Apr. 3, 1990 and U.S. Pat. No. 5,441,181 issued Aug. 15, 1995. These pistons, like other pistons known in the art, provide various trade-offs of piston movement, piston stability, ability to seal product from seeping into the pressure chamber and ability to prevent pressurized fluid from the pressure chamber leaking into the product. These trade-offs are in part affected by the nature of the product being dispensed. Product factors such as viscosity, the effect of the propellant on curing the product within the container and the requirement for product uncontaminated by propellant are co-factors in determining optimum piston design trade-offs.
Where a product, such as ice cream, is to be dispensed, the challenge is to provide a piston design which will meet the general objectives of a piston; that is, appropriate ability to move and push product during dispensing yet provide the required sealing between the top and bottom of the container in the context of a product that is loaded under pressure in a highly flowable fluid state and dispensed in a much less flowable state.
It is an object of this invention to provide a piston particularly adapted for use in a pressurized container that dispenses ice cream and similar products.
It is a related purpose to provide this function in a piston that can be readily and inexpensively molded in large quantities.